Although sodium metaperiodate (NaIO4) and periodic acid (HIO4) were useful oxidizing agents for oxidizing various organic compounds, they are comparatively expensive. Therefore, it was known a method for reusing them by oxidizing by-product, sodium iodate (NaIO3) or iodic acid (HIO3), obtained in the oxidation reaction using them to disodium paraperiodate (Na2H3IO6) using chlorine gas, isolating to collect it by filtration, and converting it into disodium metaperiodate (JP-B-2912444 etc.).